This invention relates to the optical fiber signal transmission field and, in particular, to a driver for a semiconductor laser comprising a laser, a direct current generator and current variation means.
It is a known fact that optical communication systems comprise electrooptical transmission and reception modules directly connected to the optical fiber ends which, in practice, determine the correct transfer of information and the global performance of the system.
It is also known that an optical fiber signal transmission module consists essentially of a laser, a modulation circuit which modulates the electric signal to be sent to the laser, a thermal stabilization circuit and an automatic control circuit which controls the power emitted by the laser.
It is further known that semiconductor lasers consist essentially of a P-N junction whose "excitation current v. emitted power" characteristic is similar to the "voltage v. current" characteristic of a semiconductor diode where the P-N junction must be supplied with a certain current I.sub.L in order to obtain a certain emitted power P.sub.L. This current I.sub.L is made up of a bias current I.sub.P which induces the P-N junction to work in the neighborhood of its threshold and to be ready to emit coherent light according to the laser characteristics, and of a modulation current I.sub.M which modulates the laser coherent light according to the information to be transmitted.
It is further known that temperature and aging affect the laser characteristic curve, so that in the same laser operation the knee of the curve may shift and the slope may change. It is obvious that even in the presence of such changes the laser must always emit the same power P.sub.L and it is therefore absolutely necessary to be able to change the bias current I.sub.P in order to compensate emission threshold variations and to regulate the modulation current I.sub.M in order to compensate emission efficiency losses.
It is finally known that a very important factor in the evaluation of a transmission module is its switching rate which, in practice, determines the transmission capacity and consequently the field of application of the whole optical communication system.
In a first known solution (see H. Kressel's book "Semiconductor devices for optical communication" pp. 230-232) the laser is powered by two current generators constructed with differential stages. A first generator, i.e. the bias current generator, supplies the laser with a constant current I.sub.P whilst a second generator, i.e. the modulation current generator, supplies the same laser with a variable current I.sub.M. In the time intervals in which the laser must not emit any power, the only current circulating therein is the bias current I.sub.P whilst in the time intervals in which the laser must emit power the current circulating therein is a current I.sub.L obtained from the sum of the bias current I.sub.P plus the modulation current I.sub.M.
This solution affords the advantage of allowing the regulation of both the bias current I.sub.P and the modulation current I.sub.M but also presents a variety of drawbacks. To begin with, it uses a large number of transistors involving high manufacturing costs. In addition, in order to be sure that differential stages with satisfactory characteristics are obtained, components with very narrow tolerance limits must be used since the two transistors making up the differential pair must be theoretically identical; this will further increase manufacturing costs and assembly times because of the additional checks of the component characteristics and the selection of components whose features must be as nearly identical as possible. Furthermore, the injection efficiency, i.e. the ratio between the current I.sub.L passing through the laser and the current absorbed by the modulator, ranges from 25% to 50% at the utmost, involving considerable waste. Finally, response times are rather high since both transistors of the modulation current I.sub.M generator must switch over so that, quite apart from other considerations, this solution may be rather critical for bit rates of 500 Mbit/s and above.
Other solutions were proposed in order to obtain higher bit rates ranging, for instance, from 500 Mbit/s to 1 Gbit/s. One of such solutions consists in connecting the laser in parallel with a current generator that generates a current I.sub.L which allows the laser to emit power, and with a switching device consisting of MESFET devices (see page 184 of the above-mentioned book). Although high switching rates may be obtained, this solution is practically unusable since it is impossible to keep the laser biased viewing that the current I.sub.L will either pass entirely into the laser when the switch is open or will not pass at all when the switch is closed; in addition, it is impossible to adjust the value of the current I.sub.L in such a way that the same laser-emitted power P.sub.L is always available.
Another proposed solution consists in using ECL gates (see article "Fiber optic transmitters and receivers" by M. J. Teare and L. W. Ulbricht published in the March 1985 issue of the GTE Laboratories Profile). Also this solution affords a high switching rate but the adjusting of the bias current I.sub.P and modulation current I.sub.M values so that the same laser-emitted power P.sub.L is always available is impossible.
The object of the invention is consequently that of overcoming the above drawbacks and of providing a driver for a semiconductor laser that affords a high switching rate and makes it possible, at the same time, to adjust bias current I.sub.P and modulation current I.sub.M values. Other objects include the obtaining of a very simple circuit with a high injection efficiency and which presents no particular problems regarding the choice of components.